Ostrovsky writes he was beaten again when he refused to give the password to his laptop, then kicked in the ribs by masked men who told him no one would miss him if he died.

"But as it turns out, I had it pretty easy, because I was let go," Ostrovsky writes.

Ostrovsky, who is fluent in Russian, had been working in the eastern city of Slovyansk.

Ostrovsky had been working in the eastern city of Slovyansk. Ostrovsky, who also holds an Israeli passport, was covering the crisis in Ukraine for weeks.

He says in the four nights he was detained, he saw a dozen others like him being taken in and out of the cellar of the Ukraine state security building by the militants who had taken it over.

Unlike him, many of them had been there for up to two weeks and likely are still there now, he said.

After he was released, Ostrovsky says that he learned the leader of the pro-Russia insurgents in Slovyansk told journalists they were being held as "bargaining" chips.

"I don’t yet know what he got for my release, but I hope it wasn’t very much, because no one should be allowed to take hostages no matter what their political demands are," Ostrovsky writes.

He also calls for the immediate release of anyone being held in a building controlled by the pro-Russia militants.

Since November, Ukraine has been engulfed in its biggest political crisis since the 1991 fall of the Soviet Union. Months of anti-government protests in Kiev culminated in President Viktor Yanukovych fleeing to Russia in late February.

Ukraine's acting government has accused Russia of orchestrating the unrest in eastern Ukraine, which it fears Moscow could use as a pretext for an invasion. Last month, Russia annexed Crimea weeks after seizing control of the Black Sea peninsula.